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The lighter ones tend to be a bit more sculpted, like mummy bags, but they can't be too much smaller because they need to have space to allow sleeping bags to loft within them. So a lot of them are still just basic rectangles. Or do you mean, how are they stitched, specifically? I have no idea about that aspect of construction. I passed that class but my sewing was pretty bad. :)
See a bivvy or the material?
Might not be easy to get although some people are using it small scale so someone must sell it.
https://backpackinglight.com/forums/topic/57701/
http://www.zpacks.com/materials.shtml
https://ripstopbytheroll.com/collections/all